The present invention relates to nacreous pigments having improved transparency and tinctorial strength. These are based on mica flakes coated with metal oxides.
The tinctorial strength, that is the intensity and purity of the interference color of such pigments depend critically on the quality of the metal oxide layer precipitated onto the mica flakes. The main requirement is, in addition to a homogeneous thickness of the layer, that the layer be formed crack-free and scatter light to as small an extent as possible. Cracks can arise, in particular in the case of thicker layers, when the pigments are being calcined. Moreover, it is known that not only cracks, but also, in particular, the size of the metal oxide crystals precipitated onto the mica play a critical role in determining the extent of scattering.
It is also known that the degree of scattering within the layer within the region concerned is larger, the larger the size of the individual crystals of the metal oxide precipitated. The diffuse light formed at the scattering centers within the metal oxide layer not only reduces the intensity in the glancing angle of the reflected light fraction, and hence color saturation, but also causes whitening of the interference color. In the case of crystals which are too large, the pearlescent luster and the interference color disappear completely.
Pigments prepared by the known precipitation methods, where in particular TiO.sub.2 is precipitated as a homogeneous layer onto mica flakes, are not yet satisfactory in respect of transparence and tinctorial strength.